1. Technical Field
The present invention relates to a semiconductor laser including a diffraction grating formed in the device, and to a method for manufacturing the semiconductor laser device.
2. Related Art
In fiber optic communications, a distributed feedback semiconductor laser device or a distributed Bragg reflection semiconductor laser device, which are oscillable in a single longitudinal mode, are employed mainly as light source for a middle long distance range. These semiconductor laser devices have a region including a diffraction grating formed therein, and such region of the diffraction grating provides an emission of a laser beam having a specific wave length.
Typical process for forming a diffraction grating may include an interferometric exposure process. In the interferometric exposure process, first of all, a resist is applied on a semiconductor substrate, and an exposure process is conducted with an interferometric pattern of a laser beam that is composed of divided two optical paths. In this case, a pitch of the diffraction grating can be adjusted by suitably adjusting an angle of incidence of laser beam. Subsequently, a developing process is conducted to form a resist film having a diffraction grating pattern. Further, the diffraction grating pattern is transferred onto the semiconductor substrate via a wet etch process or a dry etch process through a mask of such resist film to form the diffraction grating.
A diffraction grating formed by an interferometric exposure process described in Japanese Patent Laid-Open No. H7-170,018 (1995, p. 12, FIG. 14 and FIG. 15) is shown in FIG. 11. As shown in FIG. 11, a diffraction grating 104 is formed over the entire surface of a substrate 102.
Since the interferometric exposure process described in Japanese Patent Laid-Open No. H7-170,018 instantly achieves exposing the resist to light over a wider region on the semiconductor substrate, the formation of the diffraction grating by such process is an effective technique, in view of providing an improved throughput. Nonetheless, precise controls in a pitch of the diffraction grating and a phase shift level are difficult, and thus it is difficult to achieve higher wavelength controllability and singularity in wave mode, which are requested in the industrial area of fiber optic communications in recent years, with higher yield.
On the other hand, another process for forming the diffraction grating may be a high-resolution electron beam exposure process. In the electron beam exposure process, an improved wavelength controllability and singularity in wave mode, which are not achieved in the interferometric exposure process, can be achieved. However, in the electron beam exposure process, a dislocation may be often generated in an epitaxial layer formed on the semiconductor substrate to deteriorate a crystallinity thereof, or a crystal formulation and a thickness of a crystallized layer may be often abnormalized to deteriorate a crystallinity thereof. The region, where such crystallinity is deteriorated, is created by an influence of a bump formed in a boundary between a region having a diffraction grating on the semiconductor substrate and a region having no diffraction grating (hereinafter referred to as a diffraction grating boundary) in an etch process.
When such crystallinity-deteriorated region is included in the laser device, deteriorations in properties such as an increase of threshold current, a decrease in slope efficiency, a decreases in device life time and the like are created. In particular, when an active layer is formed on the diffraction grating, a controllability in wavelength is often reduced.
In addition, while a typical method for avoiding the deterioration of crystallinity due to the bump of the semiconductor created in the diffraction grating boundary in the etch process includes a method for forming the diffraction grating over the entire surface of the semiconductor via an electron beam exposure process, similarly as the interferometric exposure process, such method requires longer time for the exposure process, and thus a throughput efficiency is considerably reduced.
Further, a typical method for preventing a deterioration in crystallinity caused in the electron beam exposure process is a method described in Japanese Patent Laid-Open No. 2000-138,413. In Japanese Patent Laid-Open No. 2000-138,413, a method is described, in which a diffraction grating pattern is formed via an electron beam exposure and further a deep UV exposure is conducted for a region where no diffraction grating is to be formed. It is described according to such method that a bump formed between the region for forming the diffraction grating and the region for forming no diffraction grating is reduced.
However, there is a room for improvement in view of the following points in the conventional technology described in Japanese Patent Laid-Open No. 2000-138,413. First, the method additionally requires conducting the deep UV exposure process for the region for forming no diffraction grating, after the diffraction grating pattern is formed, and therefore the process may be complicated and time required for the exposure process may be increased, leading to a reduced throughput.
Second, it is often difficult to precisely conduct the deep UV exposure process only over the region for forming no diffraction grating with better controllability, such that a bump would be created between the region for forming the diffraction grating and the region for forming no diffraction grating, and thus deteriorations in properties such as an increase of threshold current, a decrease in slope efficiency, a decreases in device life time and the like are possibly created.